(a) Field of the Invention
The present invention relates to a charging system for a mild hybrid vehicle. More particularly, the present invention relates to a charging system for a mild hybrid vehicle which prevents a super capacitor from being reverse charged quickly by voltage of a battery in a case that a charging voltage of the super capacitor is lower than that of the battery.
(b) Description of the Related Art
Recently, environmentally-friendly vehicles such as hybrid vehicles and electric vehicles have attracted increased attention due to energy depletion and environmental pollution. Since hybrid vehicles have an engine as power source, hybrid vehicles do not need to charge a battery by using exterior commercial electricity. Since an electric vehicle, on the contrary, does not have the engine, the electric vehicle must charge the battery periodically by using exterior commercial electricity. In addition, the hybrid vehicle is largely classified into a mild hybrid vehicles and plug-in hybrid vehicle according to charging type. A mild hybrid vehicle charges the battery by using a portion of energy generated at an internal combustion engine, and plug-in hybrid vehicle is a hybrid vehicle that charges the battery by receiving energy from the exterior commercial electricity.
Since a pure electric vehicle and the plug-in hybrid vehicle receive the energy from exterior commercial electricity, there is a large difference between an input terminal voltage and an output terminal voltage. Therefore, an insulated buck type DC-DC converter using a transformer shown in FIG. 9 is typically used. As shown in FIG. 9, an input capacitor Ci is connected to a terminal of a high-voltage battery 102, an input of a switching element portion 106 having four switching elements Q1, Q2, Q3, and Q4 formed as full bridge is connected to the high-voltage battery 102, and an output of the switching element portion 106 is connected to a primary terminal of the transformer 108 in the insulated buck type DC-DC converter. A voltage of the high-voltage battery 102 is converted into an AC voltage by alternately turning on and off two pairs Q1-Q2 and Q3-Q4 of the switching element portion 106, and the AC voltage is dropped through the transformer 108 so as to apply a low voltage to a secondary coil. After that the low voltage applied to the secondary coil of the transformer 108 is rectified, the rectified voltage is smoothed through an inductor L and a capacitor Co and a DC voltage is charged in a battery 104. A duty ratio D for controlling the insulated buck type DC-DC converter is as follows.
  D  -                    V        LOW                    2        ×                  V          HIGH                      ×                  N        2                    N        2            
Herein, VHIGH is the voltage of the high-voltage battery 102, VLOW is a voltage of the battery 104, N1 is a winding number of a primary coil, and N2 is a winding number of the secondary coil.
Since the transformer is used in the insulated buck type DC-DC converter, efficiency is reduced due to core loss but a high-voltage side and a low-voltage side are electrically insulated. In addition, if a voltage of an output terminal is higher than that of an input terminal, reverse charging does not occur.
Because the difference between an input terminal voltage and an output terminal voltage of a charging system is small in the mild hybrid vehicle, a non-insulated buck type DC-DC converter shown in FIG. 10 is used instead of using the insulated buck type DC-DC converter which includes the transformer.
As shown in FIG. 10, the non-insulated buck type DC-DC converter 50 is disposed between a super capacitor 120 in which a voltage generated by the engine is stored and a battery 122. The non-insulated buck type DC-DC converter 50 includes a switching element 126, an inductor 132, a capacitor 134, and a free-wheeling diode 130.
The non-insulated buck type DC-DC converter 50 calculates a duty ratio from a voltage VHIGH of the super capacitor 120 being an input and a voltage VLOW of the battery 122 so as to get a target output voltage, and duty-controls the switching element 126.
The duty-control means a method that fixes a switching frequency and controls turn-on ratio in a waveform of a period. The duty ratio D of the non-insulated buck type DC-DC converter 50 is as follows.
  D  =            V      LOW              V      HIGH      
Assuming that a minimum value of the duty ratio is represented as Dmin, an equivalent impedance of loss is represented as ZL, a frequency of the switching element 126 is represented as f, an output voltage is represented as Vo, a pulsating output voltage is represented as ΔVo, a minimum inductance Lmin of the inductor 132 and a minimum capacitance Cmin of the capacitor 134 used in the circuit are as follows.
            L      min        =                            (                      1            -                          D              min                                )                          2          ⁢          f                    ×              Z        L                        C      min        =                            (                      1            -                          D              min                                )                          8          ×                      L            min                    ×                      f            2                              ×                        V          o                          Δ          ⁢                                          ⁢                      V            o                              
As known from an above equation, the inductance of the inductor 132 is inversely proportional to the switching frequency f and the capacitance of the capacitor 134 is inversely proportional to square of the switching frequency f. If the switching frequency f is long, the inductance of the inductor 132 and the capacitance of the capacitor 134 can decrease. Therefore, size of the converter can be reduced.
Because the switching frequency is made longer so as to manufacture a smaller DC-DC converter, a metal-oxide semiconductor field transistor (MOSFET) rather than an insulated gate bipolar transistor (IGBT) is widely used as the switching element. The MOSFET can be used in the circuit using high switching frequency, but is not suitable for use in the circuit using high voltage and high current. The voltage in the super capacitor is typically low (e.g., 15V-30V) but high current flows through the super capacitor in the DC-DC converter of the mild hybrid vehicle. Therefore, the circuit cannot be constructed by using only one MOSFET. Accordingly, more than two switching elements 126 are connected in parallel with each other so as to share current capacity as shown in FIG. 11.
The super capacitor 120 used in a charging system of the mild hybrid vehicle has a self-discharge circuit such that energy charged in the super capacitor 120 is discharged slowly. Since the discharged super capacitor 120 can be charged in a generating mode after the vehicle is started, the charging voltage of the super capacitor 120 becomes lowered gradually when the vehicle is not driven for a long period of time. If the charging voltage of the super capacitor 120 falls below the voltage of the battery 122, the energy charged in the battery 122 is reverse charged to the super capacitor 120 through a body diode 128 mounted at the switching element 126 of the non-insulated buck type DC-DC converter 124.
Particularly, when a power system of the mild hybrid vehicle is assembled initially, the charging voltage of the super capacitor 120 is the lowest voltage (e.g., 3V) and difference between the charging voltage of the super capacitor 120 and the voltage of the battery 122 is the largest. Therefore, when assembling the power system initially, high current flows occur. That is, if the discharged super capacitor is connected, the voltage of the battery is lowered and the charging voltage of the super capacitor becomes heightened as shown in FIG. 12.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.